Sinkholes are depressions in the ground that are linked to the collapse of an underground void. Areas that have sinkholes are known as karst terrain or karst topography. How does karst form? There are several steps, starting with the geology:


Rock type



Calcite (below left) is a mineral composed of calcium carbonate (CaCO3) – similar to the chemical in baking soda. Like baking soda, when vinegar or another acid is added, the calcium carbonate dissolves. Typically the bedrock underneath an area with sinkholes is made of limestone (below middle) which is composed of calcite. Sandstones that contain calcite (below right) also can be susceptible to sinkhole formation.

Calcite crystals (Icelandic spar variety).


Calcite crystals (above) – the main component of limestone. Photo from USGS.  Click here to read more about the mineral properties.

Photo of calcium carbonate rock through the microscope.      Photo of calcium carbonate rock through the microscope.


Photos of calcium carbonate rocks through the microscope. Left image is peloid limestone of the Smackover Formation – pinkish orange crystals are calcite. Right image is a sandstone of the Donovan Sand– white areas are quartz and feldspar grains and light pink is calcite cementing the grains together.









Rainwater is naturally acidic – not only do raindrops pick up carbon dioxide and sulfur from the atmosphere, they also become more acidic as the water interacts with vegetation. As water moves through the ground and into cracks and crevices of the underlying bedrock, the weakly acidic water dissolves the rock. As dissolving continues, the cracks and crevices become larger, forming caves and caverns.


What does a karst area look like underground? See video  

Sometimes the subsurface has small voids, and other times it may have larger voids such as caves.  Click on the movie to the right to see an example of a cave in an Alabama karst area just north of Birmingham.


Click here to go to the National Speological Society homepage to learn more about caves.








When the roof of the cave becomes too thin to support the weight of the ground above, the roof collapses. If the ground above is thin enough, this subsurface collapse can be seen at the surface (below) – this is what we see as a sinkhole.


Photo of a sinkhole next to a house.

Example of what sinkholes look like on a topographic map.

Sinkholes on a Map


Example of what sinkholes look like on a topographic map. Sinkholes and other depressions are marked by closed contour lines with hatch marks. Not always perfectly round, some sinkholes may be elongated such as the one in the upper right corner. Other sinkholes may swallow streams or be filled with water such as the one to the right of “Hollow”.






What Can Trigger a Sinkhole?


Sinkholes can form from a variety of causes including natural and man-made activities and include ground collapse related to:

§     Naturally dissolved voids in rock

§     A drop in the water table from drought or pumping of nearby wells

§     Heavy construction or weight at the ground surface

§     Drainage problems

§     Collapse of underground mines

§     Excessive rainfall


Diagram of how karst forms.




National map showing engineering aspects of karst.


Click the U.S. Geological Survey map above to see a poster on engineering aspects of karst in the U.S.


Karst  Engineering


Karst areas can be found around the U.S. and there are many techniques engineers can use to help assess sinkhole growth and voids underground.  Ground penetrating RADAR (GPR) and resistivity are two methods used to help see what is occurring under the surface. The example below shows a cross section of sinkhole area imaged by resistivity – blue areas represent voids in the rock.



Click here to read more about the use of technology in sinkhole site assessments.








National Cave and Karst Research Institute


Digital Engineering Aspects of Karst Map:  A GIS Version of Davies, W.E., Simpson, J.H., Ohlmacher, G.C., Kirk, W.S., and Newton, E.G., 1984, Engineering Aspects of Karst:  U.S. Geological Survey, National Atlas of the United States of America, Scale 1:7,500,000


Proceedings of the Second Appalachian Karst Symposium – abstracts of research presented at the 2008 symposium


Preliminary Map of Potentially Karstic Carbonate Rocks in the Central and Southern Appalachian States, by D.J. Weary, USGS, 2008




θ Read about Alabama sinkholes






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